Direct-current cable, composition and method of manufacturing direct-current cable

ABSTRACT

A direct-current cable of an embodiment includes a conductive portion; and an insulating layer covering an outer periphery of the conductive portion, the insulating layer containing cross-linked base resin and inorganic filler, the base resin containing polyethylene, a BET specific surface area of the inorganic filler being greater than or equal to 5 m 2 /g, and a mean volume diameter of the inorganic filler being less than or equal to 5 μm, the mass ratio of the inorganic filler with respect to the base resin being greater than or equal to 0.001 and less than or equal to 0.05, and the cross-linked base resin being cross-linked by a cross-linking agent containing organic peroxide.

BACKGROUND OF THE INVENTION 1. Field of the Invention

An embodiment of the present invention relates to a direct-currentcable, a composition and a method of manufacturing a direct-currentcable.

Cross-linking polyethylene cables each containing cross-linkingpolyethylene in an insulating layer covering an outer periphery of aconductive portion are widely used as alternating-current cables.

When cross-linking polyethylene, organic peroxide such as dicumylperoxide is used.

However, when a cross-linking polyethylene cable is used as adirect-current cable, volume resistivity of an insulating layer may belowered, accumulation of space charges may be increased and space-chargecharacteristics may be lowered due to cracked residue of a cross-linkingagent.

Thus, a method of forming an insulating layer containing magnesium oxideor carbon black as an inorganic filler is known (see Patent Documents 1and 2, for example).

PATENT DOCUMENTS [Patent Document 1] Japanese Laid-open PatentPublication No. 2014-218617 [Patent Document 2] Japanese Laid-openPatent Publication No. 2015-883

However, it is desired to improve long-term insulating performance of aninsulating layer against applied direct-current voltage.

SUMMARY OF THE INVENTION

The present invention is made in light of the above problems, andprovides a direct-current cable in which long-term insulatingperformance of an insulating layer against applied direct-currentvoltage and space-charge characteristics of an insulating layer aregood.

According to an embodiment, there is provided a direct-current cableincluding a conductive portion; and an insulating layer covering anouter periphery of the conductive portion, the insulating layercontaining cross-linked base resin and inorganic filler, the base resincontaining polyethylene, a BET specific surface area of the inorganicfiller being greater than or equal to 5 m²/g, and a mean volume diameterof the inorganic filler being less than or equal to 5 μm, the mass ratioof the inorganic filler with respect to the base resin being greaterthan or equal to 0.001 and less than or equal to 0.05, and thecross-linked base resin being cross-linked by a cross-linking agentcontaining organic peroxide.

According to an embodiment, there is provided a composition including:base resin, inorganic filler and a cross-linking agent, the base resincontaining polyethylene, a BET specific surface area of the inorganicfiller being greater than or equal to 5 m²/g, and a mean volume diameterof the inorganic filler being less than or equal to 5 μm, the mass ratioof the inorganic filler with respect to the base resin being greaterthan or equal to 0.001 and less than or equal to 0.05, and thecross-linking agent containing organic peroxide.

According to an embodiment, a direct-current cable in which long-terminsulating performance of an insulating layer against applieddirect-current voltage and space-charge characteristics of an insulatinglayer are good can be provided.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a cross-sectional view illustrating an example of adirect-current cable.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

Next, embodiments are described with reference to drawings.

FIG. 1 illustrates an example of a direct-current cable. FIG. 1 is across-sectional view that is perpendicular to an axial direction of adirect-current cable 1.

An outer periphery of a conductive portion 10 is covered by aninsulating layer 20 in the direct-current cable 1. Further, an innersemi-conducting layer 11 is formed between the conductive portion 10 andthe insulating layer 20 in the direct-current cable 1. Further, an outerperiphery of the insulating layer 20 is covered by a shielding layer 30,and an outer periphery of the shielding layer 30 is covered by acovering layer 40 in the direct-current cable 1. Further, an outersemi-conducting layer 21 is formed between the insulating layer 20 andthe shielding layer 30 in the direct-current cable 1.

The conductive portion 10 is formed by twisting a plurality ofconductive core wires.

As the material constituting the conductive core wire, although notspecifically limited, copper, aluminum, copper alloy, aluminum alloy orthe like may be used.

As the material constituting the inner semi-conducting layer 11,although not specifically limited, ethylene-vinyl acetate copolymer,ethylene-ethyl acrylate copolymer, ethylene-butyl acrylate copolymer orthe like may be used.

The insulating layer 20 contains cross-linked base resin and inorganicfiller.

The base resin contains polyethylene.

The polyethylene may be either of low density, intermediate density andhigh density. Further, the polyethylene may be either of straight-chainand branched.

The cross-linked base resin is cross-linked by a cross-linking agentcontaining organic peroxide.

Although the organic peroxide is not specifically limited, dicumylperoxide, 2,5-dimethyl-2,5-bis(t-butylperoxy)hexane,1,3-bis(t-butylperoxyisopropyl)benzene or the like may be used.

The base resin may further contain copolymer of ethylene and polarmonomer or polyethylene-graft-maleic anhydride. With this, the long-terminsulating performance of the insulating layer 20 against applieddirect-current voltage and the space-charge characteristics of theinsulating layer 20 can be improved.

As the polar monomer, although not specifically limited, ethyl acrylate,methacrylate, butyl acrylate, glycidyl methacrylate or the like may beused, and two or more of them may be used in combination.

The mass ratio of the copolymer of ethylene and polar monomer or thepolyethylene-graft-maleic anhydride with respect to the polyethylene is,generally, less than or equal to 1/9, and preferably, less than or equalto 5/95. With this, the long-term insulating performance of theinsulating layer 20 against applied direct-current voltage can beimproved. The mass ratio of the copolymer of ethylene and polar monomeror the polyethylene-graft-maleic anhydride with respect to thepolyethylene is, generally, greater than or equal to 0.01.

The BET specific surface area of the inorganic filler is greater than orequal to 5 m²/g, and preferably, greater than or equal to 20 m²/g. Ifthe BET specific surface area of the inorganic filler is less than 5m²/g, the long-term insulating performance of the insulating layer 20against applied direct-current voltage and the space-chargecharacteristics of the insulating layer 20 are lowered. Here, the BETspecific surface area of the inorganic filler is, generally, less thanor equal to 100 m²/g.

The mean volume diameter of the inorganic filler is less than or equalto 5 μm, and preferably, less than or equal to 2 μm. If the mean volumediameter of the inorganic filler exceeds 5 μm, the long-term insulatingperformance of the insulating layer 20 against applied direct-currentand the space-charge characteristics of the insulating layer 20 arelowered. The mean volume diameter of the inorganic filler is, generally,greater than or equal to 0.5 μm.

The mass ratio of the inorganic filler with respect to the base resin is0.001 to 0.05, and preferably, 0.005 to 0.03. If the mass ratio of theinorganic filler with respect to the base resin is less than 0.001 orexceeds 0.05, the long-term insulating performance of the insulatinglayer 20 against applied direct-current and the space-chargecharacteristics of the insulating layer 20 are lowered.

As the inorganic filler, although not specifically limited, magnesiumoxide powder, aluminum oxide powder, silica powder, magnesium silicatepowder, aluminum silicate powder, carbon black or the like may be used,and two or more of them may be used in combination.

A surface process by a silane coupling agent may be performed on each ofthe magnesium oxide powder, the aluminum oxide powder, the silicapowder, the magnesium silicate powder and the aluminum silicate powder.With this, the long-term insulating performance of the insulating layer20 against applied direct-current and the space-charge characteristicsof the insulating layer 20 can be improved.

As the silane coupling agent, although not specifically limited,Vinyltrimethoxysilane, Vinyltriethoxysilane,2-(3,4-Epoxycyclohexyl)ethyltrimethoxysilane,3-Glycidoxypropylmethyldimethoxysilane,3-Glycidoxypropyltrimethoxysilane,3-Glycidoxypropylmethyldiethoxysilane, 3-Glycidoxypropyltriethoxysilane,3-Methacryloxypropylmethyldimethoxysilane,3-Methacryloxypropyltrimethoxysilane,3-Methacryloxypropylmethyldiethoxysilane,3-Methacryloxypropyltriethoxysilane, 3-Acryloxypropyltrimethoxysilane,N-(2-Aminoethyl)-3-aminopropylmethyldimethoxysilane,N-(2-Aminoethyl)-3-aminopropyltrimethoxysilane,N-(2-Aminoethyl)-3-aminopropyltriethoxysilane,3-Aminopropyltrimethoxysilane, 3-Aminopropyltriethoxysilane,3-Triethoxysilyl-N-(1,3-dimethylbutylidene)propylamine or the like maybe used, and two or more of them may be used in combination.

Here, the inorganic filler whose surface is treated by a silane couplingagent and the inorganic filler whose surface is not treated by a silanecoupling agent may be used together in combination.

A grinding process may be performed on the inorganic filler. Forexample, a grinding process by jet grinding may be performed on theinorganic filler, whose particle size becomes larger as being adheredwith each other when performing the surface treatment using the silanecoupling agent.

The insulating layer 20 may further contain an anti-oxidizing agent.With this, thermal aging resistance of the insulating layer 20 can beimproved.

As the anti-oxidizing agent, although not specifically limited,2,2-Thiodiethylene-bis[3-(3,5-di-t-butyl-4-hydroxyphenyl)propionate],

Pentaerythrityl-tetrakis(3-(3,5-di-t-butyl-4-hydroxyphenyl)propionate),Octadecyl 3-(3,5-di-t-butyl-4-hydroxyphenyl)propionate,2,4-Bis(n-octylthiomethyl)-o-cresol,2,4-Bis(n-octylthio)-6-(4-hydroxy-3,5-di-t-butylanilino)-1,3,5-triazine,Bis[2-methyl-4-{3-n-alkyl (C12 or C14)thiopropionyloxy}-5-t-butylphenyl]sulfide,4,4′-Thiobis(3-methyl-6-t-butylphenol) or the like may be used, and twoor more of them may be used in combination.

The insulating layer 20 may further contain lubricant, a coloring agentor the like.

As the material constituting the outer semi-conducting layer 21,although not specifically limited, ethylene-vinyl acetate copolymer orthe like may be used.

As the material constituting the shielding layer 30, although notspecifically limited, copper or the like may be used.

As the material constituting the covering layer 40, although notspecifically limited, polyvinyl chloride or the like may be used.

The direct-current cable 1 may be applied for power transmission ofdirect-current power or the like.

Next, an example of a method of manufacturing the direct-current cable 1is described.

The inner semi-conducting layer 11, the insulating layer 20 and theouter semi-conducting layer 21 are formed by extrusion molding a rawmaterial of the inner semi-conducting layer 11, the compositioncontaining the base resin, the inorganic filler and the cross-linkingagent as a raw material of the insulating layer 20 and a raw material ofthe outer semi-conducting layer 21 at the same time at the outerperiphery of the conductive portion 10, and heating it to apredetermined temperature to cross-link the base resin. Next, theshielding layer 30 is formed by winding a conductive wire such as acopper tape, or an annealed copper wire around the outer periphery ofthe outer semi-conducting layer 21. Further, the covering layer 40 isformed at an outer periphery of the shielding layer 30 by extrusionmolding a raw material of the covering layer 40.

As the method of manufacturing the composition, although notspecifically limited, a method or the like may be used in which the baseresin, the inorganic filler, if necessary, the anti-oxidizing agent, thelubricant, the coloring agent and the like are kneaded to manufacturepellets, and thereafter, the cross-linking agent is heated andimpregnated to the pellets.

Here, the composition may be extrusion molded by removing aggregates byusing a screen mesh.

Further, the raw material of the inner semi-conducting layer 11, theabove described composition and the raw material of the outersemi-conducting layer 21 may be extrusion molded at the same time.

EXAMPLES

Next, examples of the invention are described. Here, a term “parts”means “parts by weight”.

Example 1

100 parts of low density polyethylene (LDPE) with a density of 0.920g/mm³, and MFR (Melt Flow Rate) of 1 g/10 min as the base resin, 0.1parts of magnesium oxide powder with a BET specific surface area of 30m²/g, and a mean volume diameter of 0.45 μm as the inorganic filler, and0.2 parts of 4,4′-thiobis(3-methyl-6-t-butylphenol) as theanti-oxidizing agent were heated and kneaded at about 180° C. tomanufacture pellets. Next, 2 parts of dicumyl peroxide as thecross-linking agent was heated and impregnated to the obtained pelletsat about 60° C. to obtain composition.

Example 2

Composition was obtained similarly as Example 1 except that the amountof the inorganic filler was changed to 1 part.

Example 3

Composition was obtained similarly as Example 1 except that the amountof the inorganic filler was changed to 5 parts.

Example 4

Composition was obtained similarly as Example 2 except that magnesiumoxide powder with a BET specific surface area of 145 m²/g, and a meanvolume diameter of 0.50 μm whose surface was treated byvinyltrimethoxysilane as the silane coupling agent was used, as theinorganic filler.

Example 5

Composition was obtained similarly as Example 4 except that 97 parts ofLDPE with a density of 0.920 g/mm³, and MFR (Melt Flow Rate) of 1 g/10min, and 3 parts of polyethylene-graft-maleic anhydride (MA-g-PE) with adensity of 0.920 g/mm³, and MFR (Melt Flow Rate) of 1 g/10 min wereused, as the base resin.

Example 6

Composition was obtained similarly as Example 5 except that 1.3 parts of2,5-dimethyl-2,5-bis(t-butylperoxy)hexane was used, as the cross-linkingagent.

Example 7

Composition was obtained similarly as Example 6 except that magnesiumoxide powder with a BET specific surface area of 30 m²/g, and a meanvolume diameter of 0.05 μm whose surface was treated byvinyltrimethoxysilane as the silane coupling agent was used, as theinorganic filler.

Example 8

Composition was obtained similarly as Example 6 except that magnesiumoxide powder with a BET specific surface area of 8 m²/g, and a meanvolume diameter of 0.2 μm whose surface was treated byvinyltrimethoxysilane as the silane coupling agent was used, as theinorganic filler.

Example 9

Composition was obtained similarly as Example 6 except that 95 parts ofLDPE with a density of 0.920 g/mm³, and MFR (Melt Flow Rate) of 1 g/10min, and 5 parts of ethylene-ethyl acrylate copolymer (poly(E-co-EA)),in which the content of units originated from ethyl acrylate was 7 mass%, with a density of 0.930 g/mm³ and MFR (Melt Flow Rate) of 4 g/10 minwere used, as the base resin, and silica powder with a BET specificsurface area of 50 m²/g, and a mean volume diameter of 0.03 μm was used,as the inorganic filler.

Example 10

Composition was obtained similarly as Example 6 except that silicapowder with a BET specific surface area of 90 m²/g, and a mean volumediameter of 0.02 μm was used, as the inorganic filler.

Example 11

Composition was obtained similarly as Example 6 except that 97 parts ofLDPE with a density of 0.920 g/mm³, and MFR (Melt Flow Rate) of 1 g/10min, and 3 parts of poly(E-co-EA), in which the content of unitsoriginated from ethyl acrylate was 7 mass %, with a density of 0.930g/mm³, and MFR (Melt Flow Rate) of 4 g/10 min were used, as the baseresin, alumina powder with a BET specific surface area of 120 m²/g, anda mean volume diameter of 0.02 μm was used, as the inorganic filler, and1,3-bis(t-butylperoxyisopropyl)benzene was used, as the cross-linkingagent.

Example 12

Composition was obtained similarly as Example 6 3.0 except that 93 partsof LDPE with a density of 0.920 g/mm³ and MFR (Melt Flow Rate) of 1 g/10min, and 7 parts of poly(E-co-EA) whose EA concentration was 7% with adensity of 0.930 g/mm³, and MFR (Melt Flow Rate) of 4 g/10 min wereused, as the base resin, and carbon black with a BET specific surfacearea of 50 m²/g, and a mean volume diameter of 0.05 μm was used, as theinorganic filler.

Example 13

Composition was obtained similarly as Example 6 except that 1 part ofmagnesium oxide powder with a BET specific surface area of 145 m²/g, anda mean volume diameter of 0.50 μm whose surface was treated byvinyltrimethoxysilane as the silane coupling agent, and 2 parts ofsilica powder with a BET specific surface area of 50 m²/g, and a meanvolume diameter of 0.03 μm were used, as the inorganic filler.

Example 14

Composition was obtained similarly as Example 6 except that 2 parts ofmagnesium oxide powder with a BET specific surface area of 145 m²/g, anda mean volume diameter of 0.50 μm whose surface was treated byvinyltrimethoxysilane as the silane coupling agent, and 3 parts ofalumina powder with a BET specific surface area of 120 m²/g, and a meanvolume diameter of 0.02 μm were used, as the inorganic filler.

Comparative Example 1

Composition was obtained similarly as Example 1 except that theinorganic filler was not used.

Comparative Example 2

Composition was obtained similarly as Example 1 except that the amountof the inorganic filler was changed to 10 parts.

Comparative Example 3

Composition was obtained similarly as Example 1 except that 2 parts ofmagnesium oxide powder with a BET specific surface area of 1.4 m²/g, anda mean volume diameter of 3 μm was used, as the inorganic filler.

Comparative Example 4

Composition was obtained similarly as Example 1 except that 2 parts ofmagnesium oxide powder with a BET specific surface area of 0.5 m²/g, anda mean volume diameter of 17 μm was used, as the inorganic filler.

Comparative Example 5

Composition was obtained similarly as Example 1 except that 2 parts ofalumina powder with a BET specific surface area of 4.1 m²/g, and a meanvolume diameter of 1.5 μm was used, as the inorganic filler.

Characteristics of inorganic fillers contained in the compositions areillustrated in Table 1.

TABLE 1 BET SPECIFIC MEAN SURFACE VOLUME AREA DIAMETER SURFACE MATERIAL[m²/g] [μm] TREATMENT 1 MAGNESIUM 145 0.5 WITH OXIDE 2 MAGNESIUM 30 0.45WITHOUT OXIDE 3 MAGNESIUM 30 0.05 WITH OXIDE 4 MAGNESIUM 8 0.2 WITHOXIDE 5 SILICA 50 0.03 WITHOUT 6 SILICA 90 0.02 WITHOUT 7 ALUMINA 1200.02 WITHOUT 8 CARBON 50 0.05 WITHOUT BLACK 9 MAGNESIUM 1.4 3 WITHOUTOXIDE 10 MAGNESIUM 0.5 17 WITHOUT OXIDE 11 ALUMINA 4.1 1.5 WITHOUT

Characteristics of the compositions are illustrated in Table 2.

TABLE 2 AMOUNT OF BASE RESIN [PARTS] INORGANIC FILLER Poly AMOUNT AMOUNTLDPE MA-g-PE (E-co-EA) NO. [PARTS] NO. [PARTS] EXAMPLE 1 100 0 0 2 0.1 —— EXAMPLE 2 100 0 0 2 1 — — EXAMPLE 3 100 0 0 2 5 — — EXAMPLE 4 100 0 01 1 — — EXAMPLE 5 97 3 0 1 1 — — EXAMPLE 6 97 3 0 1 1 — — EXAMPLE 7 97 30 3 1 — — EXAMPLE 8 97 3 0 4 1 — — EXAMPLE 9 95 0 5 5 1 — — EXAMPLE 1097 3 0 6 1 — — EXAMPLE 11 97 0 3 7 1 — — EXAMPLE 12 93 0 7 8 1 — —EXAMPLE 13 97 3 0 1 1 3 2 EXAMPLE 14 97 3 0 1 2 5 3 COMPARATIVE 100 0 0— — — — EXAMPLE 1 COMPARATIVE 100 0 0 2 10 — — EXAMPLE 2 COMPARATIVE 1000 0 9 2 — — EXAMPLE 3 COMPARATIVE 100 0 0 10 2 — — EXAMPLE 4 COMPARATIVE100 0 0 11 2 — — EXAMPLE 5 (Manufacturing of sheet)

Each of the compositions was press molded to obtain a sheet withthickness T of 0.15 mm.

Next, specific volume resistance, long-term insulating performanceagainst applied direct-current voltage and space-charge characteristicsof each of the sheets were evaluated.

(Specific Volume Resistance)

Specific volume resistance was measured by soaking the sheet in siliconeoil of 90° C., and applying a direct electric field of 80 kV/mm to thesheet using a flat plate electrode with a diameter of 25 mm.

(Long-Term Insulating Performance Against Applied Direct-CurrentVoltage)

Using the sheet, a V-t curve was obtained by soaking the sheet insilicone oil of 90° C., applying a direct electric field V₀ [kV/mm] of10 to 300 kV/mm to the sheet using a flat plate electrode with adiameter of 25 mm and measuring a period “t” [h] until dielectricbreakdown occurs in the sheet. Next, life exponent “n” was obtained fromthe formula

V ₀ ^(n) ×t=const.,

and long-term insulating performance against applied direct-currentvoltage was evaluated. Here, when “n” was greater than or equal to 20,it was determined to be double circle, when “n” was greater than orequal to 15 and less than 20, it was determined to be “0” (circle), andwhen “n” was less than 15, it was determined to be “x”.

(Space-Charge Characteristics)

Space-charge characteristics of the sheet were evaluated using a PulsedElectro Acoustic Non-destructive Test System (manufactured by Five Lab).Specifically, space-charge characteristics of the sheet was evaluated bycontinuously applying a direct electric field V₀ of 50 kV/mm to thesheet under atmospheric pressure at 30° C. for an hour, measuringmaximum electric field V₁ in the sheet, and obtaining Field EnhancementFactor FEF defined by the formula

V ₁/(V ₀ ×T).

Here, when the FEF was less than 1.15, it was determined to be “∘”(circle) and when the FEF was greater than or equal to 1.15, it wasdetermined to be “x”.

Evaluated results of the specific volume resistance, the long-terminsulating performance against applied direct-current current and thespace-charge characteristics of each of the sheets are illustrated inTable 3.

TABLE 3 LONG-TERM INSULATING SPECIFIC PERFORMANCE SPACE- VOLUME AGAINSTCHARGE RESISTANCE DIRECT- CHARAC- [Ω · cm] CURRENT TERISTICS EXAMPLE 1 1× 10¹⁵ ◯ ◯ EXAMPLE 2 3 × 10¹⁵ ◯ ◯ EXAMPLE 3 2 × 10¹⁵ ◯ ◯ EXAMPLE 4 6 ×10¹⁵ ⊚ ◯ EXAMPLE 5 8 × 10¹⁵ ⊚ ◯ EXAMPLE 6 4 × 10¹⁵ ⊚ ◯ EXAMPLE 7 7 ×10¹⁵ ⊚ ◯ EXAMPLE 8 6 × 10¹⁵ ⊚ ◯ EXAMPLE 9 5 × 10¹⁵ ⊚ ◯ EXAMPLE 10 5 ×10¹⁵ ⊚ ◯ EXAMPLE 11 6 × 10¹⁵ ⊚ ◯ EXAMPLE 12 4 × 10¹⁵ ⊚ ◯ EXAMPLE 13 7 ×10¹⁵ ⊚ ◯ EXAMPLE 14 5 × 10¹⁵ ⊚ ◯ COMPARATIVE 2 × 10¹³ X X EXAMPLE 1COMPARATIVE 1 × 10¹⁵ X X EXAMPLE 2 COMPARATIVE 1 × 10¹⁴ X X EXAMPLE 3COMPARATIVE 2 × 10¹⁴ X X EXAMPLE 4 COMPARATIVE 9 × 10¹³ X X EXAMPLE 5

From Table 3, for each of the sheets manufactured from the compositionsof Examples 1 to 13, respectively, it can be understood that thespecific volume resistance is high, and the long-term insulatingperformance against applied direct-current voltage and the space-chargecharacteristics are good.

On the other hand, as the sheet manufactured from the composition ofComparative example 1 does not contain inorganic filler, the specificvolume resistance, the long-term insulating performance against applieddirect-current voltage and the space-charge characteristics are lowered.

For the sheet manufactured from the composition of Comparative example2, as the mass ratio of the inorganic filler 2 with respect to the baseresin is 0.1, the long-term insulating performance against applieddirect-current voltage and the space-charge characteristics are lowered.

For the sheets manufactured from the compositions of Comparativeexamples 3 and 5, as the BET specific surface area of each of theinorganic filler are 1.4 m²/g and 4.1 m²/g, respectively, the specificvolume resistance, the long-term insulating performance against applieddirect-current voltage and the space-charge characteristics are lowered.

For the sheet manufactured from the composition of

Comparative example 4, as the BET specific surface area and the meanvolume diameter of the inorganic filler are 0.5 m²/g and 17 μm,respectively, the specific volume resistance, the long-term insulatingperformance against applied direct-current voltage and the space-chargecharacteristics are lowered.

(Manufacturing of Direct-Current Cable 1)

First, the conductive portion 10 formed by twisting conductive corewires made of a dilute copper alloy with a diameter of 14 mm wasprepared. Next, the inner semi-conducting layer 11 made ofethylene-ethyl acrylate copolymer, the composition as the raw materialof the insulating layer 20 and the outer semi-conducting layer 21 madeof ethylene-ethyl acrylate copolymer were extrusion molded at the sametime at the outer periphery of the conductive portion 10 to be thethicknesses of 1 mm, 14 mm and 1 mm, respectively. Then, the product washeated at about 250° C. to cross link the base resin and to form theinner semi-conducting layer 11, the insulating layer 20 and the outersemi-conducting layer 21. Next, the shielding layer 30 was formed bywinding a conductive wire such as an annealed copper wire or the likewith the diameter of 1 mm around the outer periphery of the outersemi-conducting layer 21. Then, the covering layer 40 with the thicknessof 3 mm was formed by extrusion molding polyvinyl chloride at the outerperiphery of the shielding layer 30 to obtain the direct-current cable1.

NUMERALS

-   1 direct-current cable-   10 conductive portion-   11 inner semi-conducting layer-   20 insulating layer-   21 outer semi-conducting layer-   30 shielding layer-   40 covering layer

What is claimed is:
 1. A direct-current cable comprising: a conductiveportion; and an insulating layer covering an outer periphery of theconductive portion, the insulating layer containing cross-linked baseresin and inorganic filler, the base resin containing polyethylene, aBET specific surface area of the inorganic filler being greater than orequal to 5 m²/g, and a mean volume diameter of the inorganic fillerbeing less than or equal to 5 μm, the mass ratio of the inorganic fillerwith respect to the base resin being greater than or equal to 0.001 andless than or equal to 0.05, and the cross-linked base resin beingcross-linked by a cross-linking agent containing organic peroxide. 2.The direct-current cable according to claim 1, wherein the inorganicfiller is one or more selected from a group consisting of magnesiumoxide powder, aluminum oxide powder, silica powder, magnesium silicatepowder, aluminum silicate powder and carbon black.
 3. The direct-currentcable according to claim 2, wherein a surface of each of the magnesiumoxide powder, the aluminum oxide powder, the silica powder, themagnesium silicate powder and the aluminum silicate powder is treated bya silane coupling agent.
 4. The direct-current cable according to claim1, wherein the base resin further contains copolymer of ethylene andpolar monomer or polyethylene-graft-maleic anhydride, and wherein themass ratio of the copolymer of ethylene and polar monomer or thepolyethylene-graft-maleic anhydride with respect to the polyethylene isless than or equal to 1/9.
 5. A composition comprising: base resin,inorganic filler and a cross-linking agent, the base resin containingpolyethylene, a BET specific surface area of the inorganic filler beinggreater than or equal to 5 m²/g, and a mean volume diameter of theinorganic filler being less than or equal to 5 μm, the mass ratio of theinorganic filler with respect to the base resin being greater than orequal to 0.001 and less than or equal to 0.05, and the cross-linkingagent containing organic peroxide.
 6. The composition according to claim5, wherein the inorganic filler is one or more selected from a groupconsisting of magnesium oxide powder, aluminum oxide powder, silicapowder, magnesium silicate powder, aluminum silicate powder and carbonblack.
 7. The composition according to claim 6, wherein a surface ofeach of the magnesium oxide powder, the aluminum oxide powder, thesilica powder, the magnesium silicate powder and the aluminum silicatepowder is treated by a silane coupling agent.
 8. The compositionaccording to claim 5, wherein the base resin further contains copolymerof ethylene and polar monomer or polyethylene-graft-maleic anhydride,and wherein the mass ratio of the copolymer of ethylene and polarmonomer or the polyethylene-graft-maleic anhydride with respect to thepolyethylene is less than or equal to 1/9.
 9. A method of manufacturinga direct-current cable in which an outer periphery of a conductiveportion is covered by an insulating layer, comprising: manufacturing anextrusion molded material by extrusion molding the composition asclaimed in claim 5 to cover an outer periphery of the conductiveportion; and forming the insulating layer by heating the extrusionmolded material at a predetermined temperature to cross link the baseresin.